[Fedora 15/19] kexec: Export sysfs attributes for secureboot and secure modules to user space

[Vivek Goyal]

On Thu, Sep 05, 2013 at 01:39:26PM -0400, Vivek Goyal wrote:
> On Thu, Sep 05, 2013 at 10:47:15AM -0400, Vivek Goyal wrote:
> > On Thu, Sep 05, 2013 at 02:40:39PM +0000, Matthew Garrett wrote:
> > > On Wed, 2013-09-04 at 20:49 -0400, Vivek Goyal wrote:
> > > 
> > > > I did what Eric Biederman suggested. I first unshare the mount namespace
> > > > of /sbin/kexec from parent. Then I disable any event propogation between
> > > > mounts. Then I lazy unmount existing /proc and /sys and remount them. I 
> > > > think this should make sure that we are seeing at /proc and /sys as
> > > > exported by kenrel?
> > > 
> > > Namespaces have mostly been used with the assumption that namespaces
> > > contain child processes, rather than parent processes attacking
> > > children. Are we guaranteed that (barring ptrace) a parent process is
> > > unable to manipulate a child's namespaces?
> > 
> > This is a good question and I don't know enough about namespaces to
> > be able to answer it.
> > 
> > I am CCing Eric Biederman and he should have an Idea.
> > 
> > Eric, 
> > 
> > So this is in context of kdump an secureboot. We were discussing to be
> > able to create a private mount namespace for /sbin/kexec and remount
> > /proc/ and /sys so that /sbin/kexec is sure that it is looking at
> > something as exported by kernel.
> > 
> > Is it possible for unsigned parent to now maninpulate child /sbin/kexec
> > mount namespace now? 
> 
> I am not an expert in this area. I am reading following two articles.
> 
> http://man7.org/linux/man-pages/man2/mount.2.html
> http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
> 
> I think once we did unshare() child got its own mount name space and 
> after that we converted it into a private mount point. I think that
> will ensure that no mount events are propagated from parent to child
> and vice-a-versa. I doubt that now there is any way a parent can 
> modify child's mount namespace. 

I asked this question to Al Viro. And he thinks it should be fine. He
mentioned though that make sure /proc and /sys are not symlinks. Otherwise
after I have remounted /proc, root could remove /proc symlink and replace
with directory of his choice and fake proc files in that dir.

He did mention that a new mount option to *not follow symlink* might
make sense. Initially I thought I can do stat(/proc) and if it is not
symlink then mount /proc. But I think it is racy as root could replace
/proc in between the symlink check and mount operation.

I will see if I can write a patch to introduce new mount option.

Thanks
Vivek